Multilayer circuit boards and/or wiring boards are well known in the art. The multilayer circuit boards are formed with a number of signal layers (conductive layers) which are arranged in a predetermined pattern. The signal layers are insulated from each other by dielectric layers. Thus, the multilayer circuit board is formed from interleaved (e.g., alternating) signal layers and dielectric layers.
Plated holes or “vias” are formed through the multilayer circuit board to connect one signal layer to another signal layer. The plated holes typically extend from one main side of the multilayer circuit board to an opposite side of the multilayer circuit board. In some instances, the plated holes or vias include a “stub portion” which extends away from the signal layer toward one of the sides of the multilayer circuit board. In some circumstances, it is desirable to remove the stub portion to enhance the signal to noise ratio in electronic systems that utilize high-speed signals. The smaller the stub portion the better the signal quality.
In the past, the stub portion of the plated holes is removed in a process referred to as “back drilling”. In the back drilling process, the stub portion of the plated hole is removed by drilling the stub portion to a predetermined depth. However, in practice the thicknesses of the various layers in the multilayer circuit boards are not uniform, and the depths of the signal layers within the multilayer circuit boards vary, which often changes the amount of the stub portion which needs to be removed. Drilling too deeply will disconnect the signal layer from the plated hole, or leave an unreliable connection; not drilling deep enough decreases the signal to noise ratio.
The invention discussed herein uses a technique which overcomes the problems associated with the variance in thickness of the layers in the multilayer circuit board.